Phase noise results upon mixing two free running lasers in a coherent receiver. There is a need for compensating phase noise which is usually done in a digital signal processor, DSP for short. In certain applications is not desirable to have a DSP or it might even be impossible if, for instance, an unknown analog signal is to be transmitted. Such systems are for instance antenna distribution systems or remote antenna connections to a mobile base station where digitizing the complete signal requires unreasonable data rate in the transmission link. However, transmission of an analog signal over an optical link requires a high signal-to-noise-ratio, SNR for short, a high dynamic range and a good linearity. An optical communication link which is based on quadrature modulation and coherent detection provides a superior performance in these aspects but requires compensation of the laser phase noise in order to operate properly.
Coherent detection including balanced photo detection offers superior performance in terms of receiver sensitivity, linearity and dynamic range compared to conventional intensity detection. A main problem with coherent detection is that additional laser light is required in the receiver that is mixed, i.e. multiplied, with the incoming signal to be detected. In principle the linear detection process requires the transmitter and receiver lasers to be phase locked in order to allow full recovery of amplitude and phase. Since such a locking is not possible in practice, the detected electrical signal contains significant phase noise that must be subsequently mitigated. It is noted that phase noise estimation and compensation in principle requires prior knowledge of some properties of the data transmitted, such as modulation format and baud rate, which limit the flexibility of the transmission system.
So far it has not been reported that a coherent optical link works for arbitrary signals such as radio channels from, for instance, mobile base stations or for unknown and arbitrary waveforms from, for instance, antennas or sensors. In addition, the DSP sometimes limits the performance of a phase noise mitigation algorithm due to clock frequency limitations in the DSP application specific integrated circuit, DSP ASIC for short. There are also limitations in sampling rate of the analog-to-digital converters, ADCs for short. In practice, any signal that is supposed to be transmitted over longer distances is today electronically digitized before modulated onto an optical carrier.
Nowadays coherent optical systems implement phase noise compensation in a DSP by prior knowledge about modulation format, baud rate and signal bandwidth. Such systems cannot communicate unknown and arbitrary signals unless these are first digitized and formatted for transmission over a link. However, there exist analog optical links based on intensity modulation or detection but these suffer from poor linearity and dynamic range. Such links are not suitable for long distance communication due to deterioration from chromatic fibre dispersion, CD for short. These coherent analog links do principally not mitigate phase noise and thus usually only transmit either amplitude or phase information and thus sacrifice information capacity. Further, it is difficult to recover both amplitude and phase in the analog domain. The concept of analog carrier recovery is not the best one since it requires prior knowledge of data format used. Moreover, rather complicated recovery circuits exist for achieving a demodulation carrier as well as data recovery clock.